Field of the Invention
The invention relates to a fuel cell, preferably a PEM fuel cell, and to a method for moistening the electrolyte of the fuel cell.
A fuel cell in general includes an electrically conductive current transfer plate, a cathode, an intermediate layer which conducts ions, an anode and a further electrically conductive current transfer plate, all of which are stacked on one another in the stated sequence as flat plates. Fuel cells of that construction are already known, inter alia, from the "Fuel Cell Handbook" by Appleby and Foulkes, New York, 1989 and from an article by K. Strasser entitled: "Brennstoffzellen fur Elektrotraktion" [Fuel Cells for Electrical Traction], in VDI Reports No. 912, 1992, Pages 125 to 145. Since the fuel cell can convert chemically bonded energy directly into electrical energy, it makes it possible to convert fuels such as hydrogen, natural gas and biogas, for example, into electrical energy with a higher efficiency and in a more environmentally friendly manner than it is possible to do by using the previously known conventional thermal power stations having an efficiency which is limited by the so-called Carnot's process.
As a consequence of the above-mentioned documents, a polymer-electrolyte-membrane fuel cell (PEM fuel cell) is favored in conjunction with an electrical drive. That fuel cell type can be operated both with technically pure gases and with gases and air containing CO.sub.2. The low operating temperature (&lt;100.degree. C.), the high power density, the favorable long-term behavior and the lack of a corrosive, liquid electrolyte, for example, are particularly advantageous for use in a vehicle. Corrosive liquid electrolytes are used, for example, in an acidic or alkaline fuel cell.
The water balance in the electrolyte during operation of the fuel cells represents a particular problem in the case of the fuel cells. The operability of the fuel cell is closely linked to the water content in the fuel cell and, in particular, in the electrolyte. An excessively high water content in the electrolyte leads to the available power from the fuel cell being reduced, as a result of the excessively high dilution of the electrolyte. An excessively low water content of the electrolyte likewise leads to the electrical power from the fuel cell being reduced as a result of the increase in the internal resistance. Furthermore, even in the case of the electrolyte partially drying out, gas breakdown and thus the formation of combustible gas mixtures, can occur. In the worst case, that leads to damage or destruction of the fuel cell in the event of combustion of the gas mixture.
A relatively costly vaporizer-condenser configuration has therefore already been proposed for adjusting the water content of the electrolyte in an acidic or alkaline fuel cell. In that configuration at least one of the gases flowing into the fuel cell is used to transport water vapor and, for that purpose, is also passed over a lukewarm water surface.
A PEM fuel cell, which is preferably operated with hydrogen and air, requires a vaporizer configuration for adjusting the water content in the membrane, which conducts protons. The dimensioning of the vaporizer configuration must be matched to the lowest system pressure since the maximum volume flows must be moistened in that case at a constant temperature and the largest material exchange areas are therefore required. When such a vaporizer configuration is used, the relatively large physical volume, which can reach the size of the actual fuel cell block, and the high investment costs linked thereto, must be accepted as disadvantages. When using a PEM fuel cell, those disadvantages are so serious that they preclude use, especially mobile use, and thus wider application of such fuel cells.